To increase the efficiency when harvesting with a combine harvester, the working bodies of the threshing unit, the separation unit or the cleaning unit are today increasingly controlled automatically via performance curves and/or measuring signals. To precisely adjust the process parameters and also determine the grain loss, an accurate determination of the grain output of different mechanical functional units and working bodies is extremely important. At the same time, the grain output and grain loss can be determined by means of grain output sensors in the form of structure-borne sound sensors, light barrier sensors or capacitive sensors. Structure-borne sound sensors have the advantage of being cost-effective and robust. Their operating principle is based on determining the impact energy of a kernel impacting the impact surface. This results in mechanical vibrations, which are converted to an electrical measuring signal by means of a sound converter/piezo element. The measuring signal may be evaluated in different ways in order to obtain information regarding the volume of kernels impacting the surface.
For example, the publication DE 10 2006 015 152 A1 discloses a method for grain volume determination, in which the partial surface arranged below an amplitude of a measuring signal is used as measurement for the number of kernels impacting a structure-borne sound sensor. However, the vibration amplitude depends on the grain moisture and the speed of the kernel and therefore on the temperature and moisture of the surrounding area, as well as on machine parameters. Therefore, with this method, kernels with different vibration amplitudes are evaluated differently when the grain number is detected. This results in inaccurate measurement, which are affected by environmental conditions and machine parameters.
The publication EP 0 166 137 A2 describes a method for acoustically counting particles, in which a vibration amplitude of a measuring signal measured at a structure-borne sound sensor is compared with a characteristic reference signal. If the amplitude of the measured vibration signal is smaller than that of the reference signal, a particle is counted. If the amplitude is greater the impact of a following particle is assumed. Because of the required comparison between the measuring signal and the reference signal, this method is complex. Moreover, it became apparent that it may no longer fulfills the current requirements of accuracy.